A skid steered tracked vehicle is steered by forcing opposing parallel tracks to run at different speeds (skid steering). Similarly, a skid steered wheeled vehicle is steered by forcing wheels on one side of the vehicle to run at different speeds to the wheels on the other side of the vehicle. For tracked vehicles to steer, large driving force differences are required between the two tracks—large braking forces on the inner track and high driving forces on the outer track. Differential gears and cross-shafts are used to control the relative speeds of the tracks and transfer the braking power from the inner track to the outer track to sustain the turn. A similar arrangement is used for a skid steered wheeled vehicle.
A number of electric track drive arrangements use a separate electric motor (propulsion or traction motor) to drive each track, known as a “two-line” system. The regenerative steering power in such a system is generally handled electrically resulting in the need for oversized motors and power converters to handle this power. An alternative configuration uses the same mechanical regenerative arrangement as in a conventional transmission combined with an electric drive, known as a “cross-shaft electric drive” system. In this arrangement, the steer cross-shaft runs across the vehicle outside the propulsion motor which increases the size of the assembly and requires a number of idler gears. If a gear-change is to be used, the propulsion cross-shaft should be separate from the motor shaft. This can be achieved by making the motor shaft hollow and passing the cross-shaft through the motor shaft. However, this increases the diameter of the motor bearings making a high speed motor difficult to achieve. The propulsion cross-shaft may be mounted outside of the motor, or the motor mounted outside of the propulsion shaft but this increases the package size and adds the needs for idler gears, increasing the complexity of the arrangement and reducing its efficiency.
U.S. Pat. No. 4,998,591 discloses a drive configuration which uses a single differential, mounted centrally and driven by a single propulsion motor. The differential is identical to a single differential in a conventional wheel driven car or truck axle. The torque from the drive motor is divided equally between the two half shafts which can rotate at different speeds relative to one another. On each half shaft is mounted a steer motor. To steer the vehicle, the inside steer motor must act as a brake and the outside steer motor must apply additional driving torque to generate the large track drive force difference across the vehicle to cause it to skid steer. The steer motors are operating at the speed of the half shafts, handling high torque and at high power, one regenerating and one driving and thus require oversized motors.
The present Applicant, QinetiQ Limited, has developed skid steering arrangements that make use of a controlled differential. A controlled differential has the characteristics that it couples two half shafts and controls their speeds. When the steer motor is stationary the two half shafts are simply coupled by the control differential so that they run at the same speed. When the steer motor is rotated in one direction one half shaft runs faster than the other and when the steer motor is rotated in the other direction, the other half shaft runs faster than the other. Operation of the steer motor thereby causes the vehicle to turn.
WO 02/083482 describes an electric drive configuration for a skid steered vehicle wherein a pair of propulsion motors is each in operable communication with one of a pair of rear track drive sprockets. The steer motor is in drivable communication with a controlled differential steer gear unit positioned centrally of a pair of steering cross-shafts and in drivable communication with each shaft, the opposing end of these shafts being in operable communication with one of a pair of front track drive sprockets. This arrangement enables fitting of the steering cross-shaft and controlled differential at one end of the vehicle and the drives at the other end of the vehicle, simplifying the packaging of the arrangement and reducing the volume taken up in the hull. Both track drives at the rear of the vehicle are able to continue to drive the vehicle forwards during turning unlike the two-line system where inside motors brake and the outside motors apply additional driving power to cause the vehicle to turn.
WO 02/083483 relates to another drive configuration having a controlled differential configured to cause transmission of regenerative steering powers through the propulsion motor shaft thereby removing the need for cross-shafts. A steer motor is mounted on a cross-shaft which is interconnected via gears with a shaft of the controlled differential which in turn is connected via gears and an output shaft of the controlled differential to each propulsion motor.
WO2006/0121745 describes a central casing for housing the various components of the electric drive transmission. The electric propulsion motors each comprise a stator fixed in the casing and associated with a rotor borne within the casing. A through-shaft passes coaxially through the rotor for delivering torque from each motor to track drive sprockets and a gear change mechanism is located within the casing to transmit torque from the rotor to the through-shaft at selected gear ratios. The through-shaft is rotationally supported by bearings acting between the shaft and the casing and the rotor is rotationally supported by bearings between the rotor and shaft. This enables the motor rotor bearings to be simple, small diameter low speed rated ball bearings. The arrangement also reduces the number of bearing points in the casing and the complexity, mass and cost of the casing and the overall size of the transmission can be reduced.
WO 2008/117025 describes a controlled differential for a skid steered vehicle that has a parallel pair of planetary gear sets but comprising compound (linked) planet gears in a common planet carrier coupling two motor drive shafts. Respective ring gears turn with the shafts and mesh with the compound planet gear in the planet carrier, the ratios of the teeth between each ring gear and the respective gear of the compound planet gear being unequal so that when the planet carrier is stationary the two shafts are coupled through the differential to turn together in the same sense but with a speed difference. Controlled rotation of the planet carrier varies the speed difference between the shafts in accordance with the sense and speed of rotation of the planet carrier.
The controlled differential for known skid steer transmission systems is more efficient for low speed-high torque applications. In order to minimise the control differential torque (and hence the overall weight) the rotational speed is stepped up through multiple gears in order to drive the tracks or wheels. The bearings can be highly loaded during straight-line driving as well as during steering which is not ideal. As the speed of the vehicle increases, the high speed controlled differential causes increased parasitic spin losses due to friction and aerodynamic drag. The high centripetal forces also put high stress on the bearing cages of the planet gears. The bearing stress can be addressed by the use of high strength bearing cages with anti-wear coatings but these are expensive and therefore add to the cost of the system. The heat generated is removed by the provision of cooling oil to regulate the control differential temperatures.
During a skid-steer turn, the regenerated steering power from the inside track (sprocket) is transferred through the inside final drive gear reduction, through the transmission and through the outside final drive gear reduction to the outside track (sprocket). The transmission arrangements of the prior art transfer the regenerative steering power through the transmission through four gear stages (i.e. the (inside) output gear reduction stage, the two gear sets of the steer differential, and the (outside) gear reduction stage). Given that each planetary gear stage introduces a reduction in efficiency of the energy transfer, the use of multiple gear sets leads to a reduction in efficiency of the system. The output gear stages also need to be sized to transfer the same steering power and torque as the final drive and output track sprockets, requiring large output gear stages.
Furthermore, the maximum propulsion or traction motor speed in the prior art designs is currently limited to that of the main shaft. The maximum main shaft speed is limited by the controlled differential speed and the maximum allowable load on the controlled differential planet gear bearings. For the same power output of an electric motor, the faster the motor speed can be, the smaller the size of the traction motor and associated gearing (and hence the lower the overall weight and size).
The propulsion motor speed sensors (resolvers) of the known systems are also susceptible to electro-magnetic interference due to their position around the main shaft inside the motor rotor. Additionally, there is the potential for the motor currents to cause electrical discharge machining of the gear elements, possible incompatibility of the gearbox oil with the electric motor components, and a less than optimal solution for the transmission package in terms of size, weight and efficiency; all these issues require specific remedies that complicate the design.
The present invention aims to provide novel drive configurations and controlled differentials for skid steered vehicles which seek to overcome, or at least alleviate, one or more of the aforementioned problems encountered with the prior art drive configurations.